According to the Treaty of Berlin, Eastern Rumelia was to remain under the political and military jurisdiction of the Ottoman Empire with significant administrative autonomy (Article 13). The law frame of Eastern Rumelia was defined with the Organic Statute which was adopted on 14 April 1879 and was in force until the Unification with Bulgaria in 1885.[6] According to the Organic Statute the head of the province was a Christian Governor-General appointed by the Sublime Porte with the approval of the Great Powers. The legislative organ was a Provincial Counsel which consisted of 56 persons, of which 10 were appointed by the Governor-General, 10 were permanent and 36 were directly elected by the people.

The first Governor-General was the Bulgarian prince Alexander Bogoridi (1879–1884) who was acceptable to both Bulgarians and Greeks in the province. The second Governor-General was Gavril Krastevich (1884–1885), a famous Bulgarian historian. Before the first Governor-General, Arkady Stolypin was the Russian Civil Administrator from 9 October 1878 to 18 May 1879.

During the period of Bulgarian annexation Georgi Stranski was appointed as a Commissioner for South Bulgaria (9 September 1885 - 5 April 1886), and when the province was restored to nominal Ottoman sovereignty, but still under Bulgarian control, the Prince of Bulgaria was recognized by the Sublime Porte as the Governor-General.

The earliest information on the ethnic demographics of Eastern Rumelia, before the first census was conducted, comes from ethnographic maps of the Balkans by Western cartographers. There is however little information on the actual population numbers of the different ethnic groups before 1878. According to a British report before the 1877–1878 war, the non-Muslim population (which were mostly Bulgarians) of Eastern Rumelia, was about 60% which proportion grew due to the flight and emigration of Muslims during and after the war.[9]

The results of the first Regional Assembly elections of 17 October 1879 show a predominantly Bulgarian character: Of the 36 elected deputies, 31 were Bulgarians (86.1%), 3 were Greeks (8.3%) and two were Turks (5.6%).[10] The ethnic statistics from the censuses of 1880 and 1884 show a Bulgarian majority in the province. In the discredited[11] census of 1880, out of total population of 815,951 people some 590,000 (72.3%) self-identified as Bulgarians, 158,000 (19.4%) as Turks, 19,500 (2.4%) as Roma, and 48,000 (5.9%) belonged to other ethnicities,[12] notably Greeks, Armenians and Jews. The repetition of the census in 1884 returned similar data: 70.0% Bulgarians, 20.6% Turks, 2.8% Roma and 6.7% others.[13]

The Greek inhabitants of Eastern Rumelia were concentrated on the coast, where they were strong in numbers,[14] and certain cities in the interior such as Plovdiv, where they formed a substantial minority. Most of the Greek population of the region was exchanged with Bulgarians from the Greek provinces of Macedonia and Thrace in the aftermath of the Balkan Wars and World War I. As of 2001, there are about 3400 Greeks in Bulgaria, and about 4100 Sarakatsani.

Property abandoned by Muslims fleeing the Russian army during the 1877–1878 war was appropriated by the local population. The former owners, mostly large landholders, were threatened with trial by military court if they had committed crimes during the war, so that they would not return. Two Turkish landowners who did return were in fact sentenced to death thus preventing others from desiring to come back. Those Turkish landowners who were not able to take possession of their land were financially compensated, with the funds collected by the Bulgarian peasants, some of whom were indebted as a result. For those who did return a 10% property tax was issued, forcing many to sell off their property in order to pay the tax.[16][17] Michael Palairet claimed that land rights of Muslim owners were largely disregarded despite of being guaranteed by the powers and de-Ottomanization of Bulgaria and Eastern Rumelia led to the economic decline in the region,[18] which is contradicted by many other authors, who show rapid growth of the economy as well as rapid industrial development and growth of exports in Bulgaria after 1878.[19][20][21]

^a From 1885 Eastern Rumelia was de facto part of the Principality of Bulgaria^b The western part of this canton refused to recognize the authority of Eastern Rumelia, formed the so-called Republic of Tamrash and in 1886 was ceded back to the Ottoman Empire by the Tophane Agreement^c The canton of Kardzhali was ceded back to the Ottoman Empire by the Tophane Agreement^d Burgas, Haskovo, and Pazardzhik provinces also include territory that was not part of Eastern Rumelia, while other parts of Eastern Rumelia are now in the provinces of Sofia, Smolyan and Kardzhali. The de facto independent Republic of Tamrash, which is now divided between the provinces of Smolyan and Plovdiv, didn't participate in the 1884 census.